Too high external static pressure

[racknovice]

Dear fellows:

I do mostly supermarket rack jobs. So I am not so good in commercial duct design. Here is the problem:
one of my clients wants to remodel his commercial fruit salad preparation room. He wishes pre-filters, HEPA filters and laminar flow diffusers. All of these combined with a commercial air hadling unit which uses a 0.6 KW blower motor. Checking the manufacturer's pressure drops and adding them up, I come out with a total external static pressure drop that is far greater than the air handler's allowance. What can I do to meet the required air flow? Some fellow tech said that a booster in-line tube-axial fan could be used, downstream from the AHU and upstream from the filter racks, in order to push the air through, defeating the high pressure drop. Is that right? Thank you!!

[Samarjit Sen]

Hi racknovice,

From your post it appears that your client is providing the design and the equipments. Firstly using Hepa Filters and Pre filters with laminar flow of air is a clean room application. I do not think that all these are required for a salad prepartion room.

Firstly you have to ascertain the filtration system and based on it and the duct esign you can find out the air flow and the static pressure.

More assistance can be provided if details such as the size of the room, quantity of product etc are intimated.

[racknovice]

Thank you for your time Samarjit.

You are right, my client's maybe asking too much of a system for a fruit salad preparation area, but that is what he wishes. I think he used to work for a pharmaceutical lab and wants to copy something he saw before. In another forum, a fellow tech suggested the use of an in-line tube centrifugal fan as a booster, but expressed some reserve about the usage of two fans in series. Suggested a high pressure AHU.

[KayPoh]

Hi racknovice,

You can consider using fan filter units.

[mohamed khamis]

Hi racknovice,

yes it is correct if u want to avoid the high pressure imposed on the AHU u should use two fans in series, one push the air over the AHU and the other complete the air path to pass across the filtration system.

In order to select the booster fan u should follow these steps:

1- BY knowing the required CFM for the AHU and the head pressure (let us say H1 in WTG) for the first fan (which is used to push the air over the AHU)

2- BY knowing the new pressure drop which involves the pressure drop in the AHU and the new components of the filtration system.

3- The total new head presure for the system is the product of the new pressure drop divides the specific gravity of water (9800) with caring of the unit of the psychical quantities (let us say the total new head pressure will be H2 in WTG)

4- The head pressure of the booster fan equals to (H2 - H1), Go to the booster fan performance curves and select the suitable fan specifications corresponding to the head pressure (H2 - H1) and the required CFM. Then u can get the needed fan specifications. For the installation purpose downstream from the AHU and downstream from the Filter racks.

I wish it could help

Cheers

[Samarjit Sen]

Hi racknovice,

If you can provide me with the dimension of the room and the correct air volume being discharged from the air handling unit, the length of the ducts and if any filtration media has already been installed, I will try and give you a solution. Please also let me know as to where are air out lets located.

Please at the same time try to convince your client that this is not a Pharmaceutical Project where you need a clean room. In this case you have to ensure proper hygene. Has he also proposed any clean room classification for his requirement.

[TXiceman]

There are a number of companies that build high static pressure air handlers for applications like yours. When you go to high filtration units, you can expect to deal with high static fans.

You need to look at the static on the system with dirty or loaded filters as well as clean.

One company that could possibly help you is www.RAE-corp.com.

Ken

[hamidpia]

Hi racknovice
I am very late in posting reply, nevertheless, you need to have separate AHU with fan of high ESP and this AHu will serve only Salad area nothing else because you are using very clean air for this area and recirculation should not be mixed with less clean air , otherwise your HEPA filter and pre filters will get choked very early. More air will tend to flow area of less resistance if oneAHU is used. If it is single pass 100% fresh air then using booster fan shall serve the purpose. High static pressure AHU is not a big deal.

[vijay_study]

Hi there,

I would like go suggest following points:

1. First of all if your client really wants HEPA filters, you will require a 5micron filter before the HEPA filter section in order to have longer life of the HEPA filters.

2. Secondly check the external static pressure of the existing air handling unit and the type of blower.

Check the blower curves for existing static pressure and blower curves for revised static pressure. If it lies within range you can select same blower with higher capacity motor in order to obtain required static pressure.

3. If the blower is backward curved, you wont have problems with surging. However if the blower is forward curved check for the problem of surging.

4. Fan filter units may lead to balancing problems if the are is pressurised, otherwise it is a good option.

5. You can also install a plenum comprising of 5micron and HEPA filter section, just downstream of the air handling unit if your existing blower caters to required capacity.

Regards

Vijay Kumar